Sheath flow is a particular type of laminar flow in which one layer of sample fluid, or a particle, is surrounded by another layer of focusing fluid on more than one side. The process of confining a particle stream in a fluid is referred to as a ‘sheath flow’ configuration. For example, in a sheath flow configuration, a sheath fluid may envelop and pinch a sample fluid containing a number of particles. The flow of the sample fluid containing particles suspended therein may be narrowed almost to the outer diameter of particles in the center of the sheath fluid. The resulting sheath flow flows in a laminar state within an orifice or channel so that the particles are aligned and accurately pass through an orifice or channel in a single file row.
Sheath flow is used in many applications where it is preferable to protect particles or fluids by a layer of sheath fluid, for example in applications wherein it is necessary to protect particles from air. For example, in particle sorting systems, flow cytometers and other systems for analyzing a sample, particles to be sorted or analyzed are usually supplied to a measurement position in a central fluid current, which is surrounded by a particle free liquid sheath.
Sheath flow is useful because it can position particles with respect to sensors or other components and prevent particles in the center fluid, which is surrounded by the sheath fluid, from touching the sides of the flow channel and thereby prevents clogging of the channel. Sheath flow allows for faster flow velocities and higher throughput of sample material. Faster flow velocity is possible without shredding cells in the center fluid because the sheath fluid protects the cells from potentially high shear forces at the walls of the flow channel.
Conventional devices that have been employed to implement sheath flow have relatively complex designs and are relatively difficult to fabricate.